As a typical example of a display device, a liquid crystal display device and an organic EL (electroluminescence) display device respectively having a liquid crystal element and a light-emitting element in each pixel are represented. These display devices possess a liquid crystal element or an organic light-emitting element (hereinafter, referred to as a light-emitting element) in each of a plurality of pixels formed over a substrate. A liquid crystal element and a light-emitting element have a layer including a liquid crystal and an organic compound, respectively, between a pair of electrodes and are driven by applying a voltage or supplying a current between the pair of electrodes.
Unlike a liquid crystal element, current flows in an organic compound included in a layer (hereinafter, referred to as an EL layer) sandwiched between a pair of electrodes in a light-emitting element. Thus, the organic compound is oxidized or reduced during operation of the light-emitting element and may exist in a state having an electric charge. Furthermore, an excited state is generated by recombination of these active species. Such active species and molecules in an exited state readily react with another organic compound or impurities such as water and oxygen, which enter a light-emitting element, due to their higher reactivity than that of a molecule in an electrically neutral state or a ground state. These kinds of reactions adversely influence performance of a light-emitting element, resulting in reduction of efficiency and lifetime of a light-emitting element.
As a method for suppressing the aforementioned reduction in performance, it is disclosed in Japanese patent application publications No. 2014-154450 and 2009-266922 that a passivation film (sealing film) is formed over a light-emitting element. Since entrance of impurities to a light-emitting element can be prevented or suppressed by the sealing film, it is possible to prevent or suppress the reduction of efficiency and lifetime of a light-emitting element.